Portable communication devices, such as cellular telephones, use one or more power amplifiers to amplify an information signal prior to transmission. The power amplifiers are typically implemented as one or more stages of transistors and related circuitry. In most applications, the operating point of the transistor is set by providing a bias current or voltage to one of the terminals of the transistor. In the case of a bipolar junction transistor (BJT) the bias current is normally applied to the base terminal of the transistor to control the parameters under which the transistor will conduct between its collector and emitter terminals. A typical bias circuit should be able to provide the desired bias conditions (i.e., current) over a wide supply and battery voltage range. In the past, a regulated constant voltage has been used as the source of the bias signal, thus creating a fixed bias condition.
However, since the gain, linearity and efficiency of a power amplifier are very sensitive to the bias conditions of particular transistors in use, it is a technical challenge to maintain an optimal bias point at different supply voltages in order to maximize the overall performance of the power amplifier.
Traditional bias circuits have used what is referred to as a current mirror to provide a fixed bias current over a supply voltage range. The bias condition is determined by the particular bias profile at a given supply voltage, unless a controllable reference voltage supply is used to change the bias condition. However, such an approach is costly and complex because it requires additional circuitry to provide the reference voltage, thereby increasing the circuit complexity. Because of the fixed current profile in existing bias circuits, the performance of the power amplifier is optimized at only the nominal supply voltage condition. This situation results in degraded power amplifier performance at other supply voltages.
Furthermore, even when the power amplifier is switched off there are many active transistors in the power amplifier circuit that are not completely turned off, thus leading to leakage current flow in the device. This shortens standby time for the device in which the power amplifier is implemented.
Therefore, it would be desirable to be able to bias a power amplifier over a wide range of supply voltage and also be able to minimize the leakage current of the transistors in the power amplifier in order to improve standby time.